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LIGHTING OILS                                    347

EXAMPLE :   During 5 hours the following intensities were observed, the
consumption of oil being 180 grams.

ist hour
2nd    ,,
3rd    „
4th    „
5th    ,,

Luminous intensity in


. 8-50

Total candles   42-50

Mean luminous intensity = 42-50 : 5 = 8-50 candles.

Mean consumption per candle-hour = 180 : 42-50 = 4-23 grams.

Yield, or light produced per gram of oil = 42-50 : 180 = 0-23 candle-hour.

Decrease of luminous intensity = (Q^O-^^O) : 9-50 = 0-21.

It must be pointed out that petroleums of different quality do not burn
equally well in all lamps, principally on account of the different quantities of
air they require to burn completely. If the lamp does not allow access of
sufficient air the combustion, being incomplete, will give rise to smoke and
unpleasant smell, whereas excess of air in the flame will cool the latter too much
and diminish the luminosity as well as the consumption. In general, Russian oils
require more air than the American, while with oils from the same locality,
more air is required by those rich in heavy and poor in light fractions. In any
case it is necessary, to obtain comparable results, to work with the same lamps
and to allow for the form and dimensions of their essential parts, namely, the
holder, burner and chimney. Marked influence on the course of the combus-
tion is also exercised by the wick, especially the length and quality of the fibre
and the structure and compactness of the tissue; wicks of the same quality
and dimensions should be used in all measurements, and they should be either
new or washed thoroughly with petroleum ether and then dried.

8. Behaviour at Low Temperature.—This test is made on oils to
be used in the open in cold places. A little of the oil is cooled for an hour,
in a test-tube with a thermometer passing through its stopper, at the lowest
temperature to which it is likely the oil may be exposed, to ascertain if
the oil remains clear and mobile or if solid substances separate. For the
procedure, see Heavy Oils, Physical Tests, 8.

2. Chemical Tests

1.  Acidity.—This may be due to inorganic acids (principally sulphuric
acid) or to organic acids.    The tests are made as follows :

(a) INORGANIC ACIDS. The oil is shaken with tepid water containing
a little methyl orange ; if the colour changes to red, the aqueous layer is
separated and tested with barium chloride.

(&) ORGANIC ACIDS. If test (a) gives a positive result, the oil to be
used for the present test is first washed with hot water: 100 c.c. of
the oil are dissolved in 100 c.c. of a neutral alcohol-ether (i: 4) mixture
in presence of a drop of phenolphthalein solution and a drop of N/io-sodium
hydroxide solution, the whole being shaken in a cylinder: the red colour
persists if the oil is neutral, but disappears if organic acids are present.

2.  Degree of Refining.—The oil is shaken with an equal volume of, more than one-half distils above 90°, these